PROJECT 3 ABSTRACT Diarrheal disease remains a leading cause of childhood illness and mortality worldwide and is especially life threatening in infants and children under 5 years when the immune system is developing. The gut microbiome begins its development during this same critical window, and its interaction with adaptive and innate immunity is required to achieve immune competence. Although vaccines have helped to reduce the population burden of gastrointestinal infections, the magnitude of individual-level protection conferred by immunization varies between children. This project seeks to test the hypothesis that the structure of bacterial communities residing in the gut may be an important ? and potentially modifiable ? factor influencing infants' abilities to mount a beneficial immune response to vaccines and disease risk. We will leverage data from the unique, prospective New Hampshire Birth Cohort Study (2009 to present) that collects detailed epidemiological (e.g., gestational age, delivery mode, medical records, antibiotic exposures, infections), lifestyle (e.g., maternal diet, smoking, probiotic intake, infant feeding practices), and biospecimen (e.g., maternal peripheral blood, cord blood, stool samples) from maternal-child dyads recruited during pregnancy and followed through the early childhood period. The preliminary clinical endpoints to be investigated include infants' rotavirus mucosal antibody concentrations in stool samples and gastroenteritis occurrence during the first year of life. In Aim 1, we will examine the associations of putative disruptors of the early life microbiome assembly (i.e. cesarean deliveries, antibiotics, and formula feeding) and infant vaccine response and gastroenteritis risks. In Aim 2, we will compare how the community structure, alpha-diversity level, and relative abundance of specific bacterial taxa in the intestinal microbiome relate to infant vaccine response and gastroenteritis risks. In Aim 3, we will implement a novel approach for investigating the role of microbiome in mediating associations between perinatal and early postnatal exposures and infant's and mucosal antibody response and risk of gastroenteritis. Determining the role of the infant gut microbiome and the factors that influence the microbiome in immunogenic vaccine responses will help us to identify key elements of microbial colonization patterns in relationship to immune training in infants, as well as identify targets for interventions designed to promote vaccine effectiveness and reduce the burden of infant infection.